basic nw complete_chandan

7/27/2019 Basic Nw Complete_Chandan

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Welcome

Lt Col Chandan Tiwari


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DATA COMMUNICATIONAND

COMPUTER

NETWORKS

INTRODUCTION


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Data comn are the exchange of data b/w twodevices via some form of txn medium such aswire cable.

These comn devices must be part of a comn sys(combination of HW & SW)

Effectiveness of data comn depends upon Delivery

Accuracy

Timeliness

Jitter



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Source

Generates data to be transmitted

Transmitter

Converts data into transmittable signals Transmission System

Carries data

Receiver

Converts received signal into data Destination

Takes incoming data



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Message

Sender

Receiver Txn Media

Protocol



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Simplex

Half - Duplex

Full - Duplex



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Simplex



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Half-Duplex



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Full-Duplex



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data flows move in one direction only, (radio or

cable television broadcasts)

data flows both ways, but only one direction at

a time (e.g., CB radio) (requires control info)

data flows in both directions at

the same time



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NETWORKS



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A network is a set of devices(referred as

nodes) connected by comn ch.

Node can be any device capable of sending/

receiving data generated by other nodes on

the network.



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Computer Network

Computer Network is a collection of autonomous

computers interconnected by a single technology. eg

Ethernet LAN

Is Interneta computer network?



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Distributed System is a collection of

independent computers that appear to its

users as a single coherent system

Existence of multiple autonomous computers is

transparent

It is like a virtual uniprocessor

Implemented in SW and build over computernetworks



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Performance

Throughput (High)

Delay (Low)

Reliability

Security



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Computers connected over a network canmake that information exchange easier andfaster.

The information moves directly fromcomputer to computer rather than through ahuman intermediary.

People can concentrate on getting their workdone rather than on moving informationaround the company.



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Resource Sharing

Robustness

Load Balancing Location Independence

Productivity



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Based on transmission technology

Broadcast networks

Pt-to-Pt networks

Based on scale Personal Area Networks (PAN)

Local Area Networks (LAN)

Metropolitan Area Networks (MAN)

Wide Area Networks (WAN)



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Pt to Pt

Broadcast

Topology Based Cct SW Vs Message SW Vs Packet Sw



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Broadcast Networks

Single comn channel shared by all cmptrs

Packets send by one cmptr received by all others.

Address in packet- specifies for whom intended.

Packet can also be addressed to all cmptrs (broadcast)

or a gp of cmptrs(Multicasting)

Smaller networks broadcast

Eg Ethernet(802.3),IBM Token Ring(802.5)



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Single comn channel shared by all cmptrs on

NW.

Packets sent by one cmptr received by all others. Address fd in packet- specifies for whom

intended.

Packet can also be addressed to all cmptrs(broadcast).

Some networks also support multicasting



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Point To Point Networks

Many connections between indl pairs of cmptrs.

Packets visit one or more intermediate machines.

Multiple routes .

Routing algorithm To determine the best route

Also called as unicasting

Larger networks pt to pt

Eg. WAN



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Many connections between indl pairs ofcmptrs.

Packets visit one or more intermediate

machines. Multiple routes.

Routing algorithm.

Smaller networks broadcast larger networks pt to pt



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LAN

WAN

MAN



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A local area network (LAN) is a number ofcomputers connected to each other bycable in a single location by a common

medium i.e switch, hub and etc, usually asingle floor of a building or all thecomputers in a small company within thegeographical area.



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Local Area Networks(LAN)

Within same building/ campus - upto a few kms.

Speed 4/16/10/100/1000 Mbps.

Normally broadcast type

Topology (bus,ring, star) Restricted in size- worst case txn time is bounded and

known in advance.eg 10 Mbps Ethernet LAN(10

BaseT) has max dia of 500m Simple network management

Eg- Ethernet(802.3),IBM Token Ring(802.5),Wi-

Fi(802.11,Wireless LAN)Lt Col Chandan Tiwari


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Operates within the limited geographical area.

Allow access through high bandwidth up to 1000

mbps.

Controls the network under local administration

Provides the full time connectivity to local system.

Connects physical adjacent devices.



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Metropolitan Area Networks(MAN)

Bigger version of LAN.

Uses similar technology as LAN.

Can cover a city.

Eg- Wi-Max(802.16,Wireless MAN)



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While local area networks are perfect for sharingresources within a building or campus, they cannot beused to connect distant sites.

Wide area networks (WANs) fill this need. Stated

simply, wide area networks are the set of connectinglinks between local area networks.

These links are made over telephone lines leasedfrom the various telephone companies. In rareinstances, WANs can be created with satellite links,

packet radio, or microwave transceivers. These options are generally far more expensive than

leased telephone lines, but they can operate in areaswhere leased lines are not available.



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A wide area network links computers in differentlocations.

M C E M ESecunderabad

ARMY HQ

New Delhi



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Operates over a large geographical area.

Allow access over serial interface

Works at a local speed 2mbps is maximum speed

in internet.

Connects devices separates wide even global area.


Wide Area Networks(WAN)


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Wide Area Networks(WAN)

Spans larger geographical area.

Collection of cmptrs (hosts) connected by a comn

subnet

Subnet consists of :-

Transmission lines.

Switching elements (or routers).

Packet sent from router to router. (Store and fwd /

packet switching)

Gen Pt to Pt Topologies



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Star

Mesh

Ring

Bus

Hierarchical

To reduce complexity of NW

Better comd & contrl Better mgmt

Tiered Architecture



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Point-to-Point Line Configuration



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Multipoint Line Configuration



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Mesh Topology



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Star Topology



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Tree Topology



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Bus Topology



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Ring Topology



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Hybrid Topology



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Baseband



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Broadband



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Bandwidth is the capacity of a medium to conveydata.

One example of bandwidth is automobile traffic. Atwo-lane road with a speed limit can accommodate

only so many cars before there are too many and atraffic jam results.

You can increase the bandwidth of a road by

making the cars travel more quickly (which corresponds

to using a faster transmission method in networks)or

by making the road wider (which corresponds to usingmore wires in networks).



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Means sending a digital signal over channel w/ochanging digital signal to an analog signal.

The cable connecting the computer can carry onesignal at a time, and all the system take turn using it.

This type of network is called Base band network. In the base band network, when a computer transmits

data it might be broken into many packet andtransmits separately.

The receiving system reassembles them back intooriginal. This is called packet switching network.



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The alternative to a packet switching network is cct

switching. In CSN two system established a cct before

communication and broken cct only after the finish the

communication between them.

HELLO HOW ARE YOU

YOUAREHOWHELLO



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VOIP FTP Telnet

Co-ax Wireless OFC



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Single comn channel shared by all cmptrs onNW.

Packets sent by one cmptr received by allothers.

Address fd in packet- specifies for whomintended.

Packet can also be addressed to all cmptrs

(broadcast). Some networks also support multicasting



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Computer NWs are often described using

layered architecture

Layered architecture specifies functionality at

each layer (modularity)

Higher layer protocols can operate without

knowing details of the lower layers .



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Fundamental idea is to provide a service but

keep details of implementation i.e. internal

state and algorithms, at each level hidden.

Info hiding.

Abstraction

Data encapsulation.



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Layering hides information

If it did not (layering violation) then changes to

one layer could require changes everywhere

Sometimes hiding information can degrade

performance



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Example. flow control protocol at an upper layer may think

packet loss is always because of network congestion

But if it is due to a lossy link then performancedegrades.

So hiding information about reason of packet loss

from flow control protocol leads to degraded

performance



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Conflict between information-hiding andachieving good performance

Leak enough information to allow good

performance but not so much that smallchanges in one layer need changes to other

layers



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Aprotocolis a set of rules and formats thatgovern the communication between peers

set of valid messages

meaning of each message

A protocol is necessary for any function that

requires cooperation between peers



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Problem: Exchange a file over a network that

may corrupt packets but doesnt lose or reorderthem

A simple protocol

send file as a series of packets send a checksum

receiver sends OK or not-OK message

sender waits for OK message if no response, resends entire file



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Switching



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Packet

Switch



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A network is a set of connected devices.

Prob how to connect them for one to one

comn

Pt to Pt

Different topology

Switching

SW NW consists of a series of interlinked nodes,

called switches.

SW are capable of creating temp conn b/w two

or more devices linked to SW.



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The end devices are called stations. The switching devices are called nodes.

Key features of a switched communicationnetwork Network Topology is not regular. Uses FDM or TDM for node-to-node communication.

There exist multiple paths between a source-destination pair for better network reliability.

The switching nodes are not concerned with thecontents of data.

Their purpose is to provide a switching facility thatwill move data from node to node until they reachthe destination.



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SwitchedNetworks

CircuitSwitched NW

PacketSwitched NW

DatagramNW

VirtualCircuit NW

MessageSwitched NW



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circuit switching implies that there is adedicated communication path between the

two stations.

The path is a connected through a sequenceof links between network nodes.

On each physical link, a logical channel is

dedicated to the connection.



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Circuit SW takes place at the physical layer Before starting comn, the stn must make a reservation of

resources. BW in FDM / time slots in TDM

SW Buffers

SW processing time SW input/output port

Data transfer are not packetized (Physical layer transfer),data are continuous flow

No addressing involved during data transfer SWs route the data based on their occupied band(FDM) ortime slot (TDM)



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Efficiency Low

Delay Minimal

No waiting at SWs Total delay = Time to est conn + transfer Data +

Disconnect the circuit

Data propagation delay 5msec per 1000km

No congestion

No Busy signal



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OSI Reference Model


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International Organisation for

Standardisation (ISO) -international organisation

responsible for a wide range

of standards.

1984 - Open Systems Interconnect ion (OSI)

Reference Modelapproved as international

std for comn architecture to aid NWinterconnection without necessarily requiring

complete redesign.



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7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical



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7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

NIC Card

Hub



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Provides network services to

application processes (such as

electronic mail, file transfer, and

terminal emulation)

7 Application



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7 Application

6 Presentation

5Session

Transport4

Network3

Inter-host communication

Network services to applications

Data representation

End-to-end connection reliability

Addresses and best path Identifying source and destination

Path selection between two

systems (routing)



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7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2



Data representation


Addresses and best path

Access to media

Provides reliable transfer of dataacross media

Physical addressing, network

topology, error notification, flow

control



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7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2

Physical1



Data representation


Addresses and best path

Access to media

Binary transmission

Wires, connectors, voltages,

data rates



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Application

Presentation

Session

Transport

Network

Physical

Data Link

Application

Presentation

Session

Transport

Network

Physical

Data Link

Host A Host B

Data} {



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message

segmentdatagram

frame

sourceapplication

transportnetwork

linkphysical

HtHnHl M

HtHn MHt M

M

destinationapplicationtransportnetwork

link

physical

HtHnHl M

HtHn M

Ht M

Mnetwork

linkphysical

link

physical

HtHnHl MHtHn M

HtHnHl MHtHn M

HtHnHl M HtHnHl M

router

switch

Data Encapsulation



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Layer 2

Data Link Layer



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Reliable communication over a single link.

Introduces the notion of a frame

set of bits that belong together

Begin and end markers delimit a frame



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On a broadcast link (such as Ethernet) end-system must receive only bits meant for it

need datalink-layer address

also need to decide who gets to speak next these functions are provided by Medium

Access Sublayer



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DLL protocols are the first layer of SW &heavily dependent on underlying physical

link properties.

Hence, both physical and data link layers

are usually bundled together on Network

Interface Card



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24 bits

ROM

RAM

24 bits

2b5f.0c12. 3a56

Serial NumberVendor Code

MAC address is burned into ROM on a networkinterface card

Can it be changed? (OTW)



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Layer 3

Network Layer



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Breaks Transport Layer PDUs into

packets and ensures their delivery

Defines logical addressing

Responsible for routing on the NW

(Routers operate at this layer)

Found both in end-systems and in

intermediate systems



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Network Node

1 1GeneralExample

Network Host

10. 8.2.48TCP/IPExample

(Mask 255.0.0.0)

Network Node

1aceb0b. 0000.0c00.6e25Novell IPXExample



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BB

X Y

AA

CC

Presentation

Data Link

Physical

Data Link

Physical

Router A Router B Router C

Data Link

Physical

Data Link

Network

Transport

Session

Presentation

Application

Physical

Host X Host Y

Data LinkNetwork

Transport

Session

Application

Physical

Network Network Network



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AA BB

CCDD

XX

Can an alternate route substitute for a failed route?

Yes but With dynamic routing enabled



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Transport Layer

Layer 4



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Transport layer Multiplexes multiple applications to the same end to end


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Multiplexes multiple applications to the same end-to-end

connection

adds an application-specific identifier (port number) so that receivingend-system can hand in incoming packet to the correct application



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Transmit

Buffer Full

Not ReadyStopProcess

Segments

Buffer OKReadyGo

Resume Transmission

ReceiverSender



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Not common

Establishes, manages, and terminatessessions between applications

Provides full-duplex service, expedited

data delivery, and session synchronization



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Duplex

if transport layer is simplex, concatenates twotransport endpoints together

Expedited data delivery

allows some messages to skip ahead in end-system queues, by using a separate low-delay transport layer endpoint

Synchronization

allows users to place marks in data streamand to roll back to a pre-specified mark



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Presentation Layer

Layer 6


U lik th l hi h d l ith h d


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Unlike other layers which deal with headers,

presentation layer also touches theapplication data

Hides data representation differences

between applications. For example: Endian-ness Characters (ASCII, unicode, EBCDIC.)

Can also encrypt data

Compression



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Application Layer

Layer 7


Closest to the user


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Closest to the user.

Provides network services to the users applications. Application layer establishes the availability of intended

communication partners, synchronizes and establishesagreement on procedures control of data integrity.

Differs from other layers in that it does not provide

services to any other OSI layer, but rather, only toapplications outside the OSI model.

Examples of such applications are Spreadsheetprograms, Network virtual terminal, Mail services,Directory services, File transfer, access, and

management.



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TCP/ IP REFERENCEMODEL



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Research sponsored by United States DoD Major design goals

Ability to connect multiple NWs seamlessly

Survivability

Flexible architecture to cater for applications with divergentrequirements

DoD wanted connections to remain intact as long as source

and destination machines were intact.



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Connectionless packet switched service Uses Internet Protocol (IP) which defines packets

Permit injection of packets into any NW and have them

travel independently to the destination

Packet routing and congestion control are major issues.

Similar to OSI Network Layer .



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Designed to allow conversation between peer entities on

source and destination hosts Similar to OSI Transport Layer.

Two protocols defined for this layer:

Transmission Control Protocol (TCP) - handles flow control and

sequencing. Reliable connection oriented protocol

User Datagram Protocol (UDP) prompt delivery more importantthan accurate delivery. Unreliable connectionless protocol



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No session or presentation layer.

Application layer serves as the communication interfacefor users by providing specific application services to theuser

Protocols - Virtual terminal (telnet), FTP, SMTP, DNS,HTTP, etc.


TCP/IP f d l d l d d h OSI


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TCP/IP reference models development preceded the OSI

model by several years. Unlike OSI, TCP/IP was never intended to be an

international standard.

TCP/IP was not designed with layers and does not fit neatly

into OSI models 7 layers.

OSI introduces the concept of Services, Interfaces and

Protocols. TCP/IP model originally did not distinguish

between these.


P t l b tt hidd i OSI d l d th il


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Protocols better hidden in OSI model and thus easily

replaced with change in technology

In OSI model, protocols were invented after model was

devised. In TCP/IP model, protocols came first and model

was merely description of protocols



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ADDRESSES



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MAC SUB LAYER



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Static allocation The channel BW is divided equally among users.

Unused bandwidth will be lost.

Dynamic allocation

Users able to access unused bandwidth from

others.

There is no dedicated bandwidth.


Traditional method example - Frequency Division


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Traditional method example Frequency Division

Multiplexing (FDM). Involves splitting up the usable frequency into

smaller channels.

Unused bandwidth will be wasted, but the allocatedbandwidth is guaranteed.

Simple and efficient for known workloads.


Larger the number of users, smaller are the FDM


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channels.

In computer systems, data is generally bursty, thus,

FDM results in poor bandwidth utilization as stn is

idle for most of the time.

Data bursts from one computer will take a long time

to travel over the bandwidth, while other computers

may not be using their resources.

Similar is the case for Time Division Multiplexing

(TDM)


Key assumptions:


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Key assumptions:

Station Model

The network consists of independent stations (orcomputers).

All stations are considered to be equal.

They produce frames to be transmitted.

Once a frame is generated the station is blocked

and does nothing until the frame has beensuccessfully transmitted.



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Single Channel Assumption There will be a single channel for all of the

terminals to communicate on.

All stations can transmit and receive on it.



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C i S


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Carrier Sense

The terminal can sense if the channel is busy No terminal will transmit until the channel is idle

No Carrier Sense

The terminal cannot sense if the channel is busy

Terminals will transmit and check for collisions later.

LANs generally use Carrier Sense but not satellitenetworks. Why?


If more than 2 users send at the same time - collision


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All collided packets are lost -> waste of bandwidth

Ideally, the MAC protocol for a broadcast channel with

the bit-rate R bpsshould satisfy:

if only 1 node is sending then the throughput is R

when M nodes have data to send than the

throughput is R/M

decentralized protocol no master

simple & inexpensive to implement



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Carrier Sense Multiple Access Protocols (CSMA) Collision-Free Protocols

Wireless LAN Protocols



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Stn monitors the channel until it is idle.


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Once it is idle, the Stn transmits.

If a collision occurs the stn waits for a random amount of timeand starts all over again.

Known as 1-persistent because the probability that it willtransmit on an idle channel is 1.



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Longer is the propagation delay, worse is theperformance of the protocol


Limitations of 1 persistent protocol


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Propagation Delay.

Simultaneous Txn will result in collisions.


Before sending Node senses the channel. If idle it


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transmits its frame. If channel is busy, the node waits for random time

and then repeats the algorithm.

Less greedy than 1-persistent protocol which

continues to sense the channel to try to seize itimmediately upon detecting the end of previous

transmission.



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Comparison of the channel utilization versus

load for various random access protocols.Lt Col Chandan Tiwari

Stations abort transmission on detecting collision asopposed to continuing to send out data that will be


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opposed to continuing to send out data that will be

lost anyway. In CSMA, each collision wastes at least one complete

frame time, because both stations transmit theircomplete frames even though they are garbled theinstant they collide.

Terminating the transmission on detecting collisionsaves both time and bandwidth.

The transmitting stns will then wait a random

amount of time and try again.

Used in Ethernet LAN.



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Collision detection can take as long as 2. A stn cannot be sure

that it has seized the channel until it has transmitted for 2

without hearing a collision Lt Col Chandan Tiwari

802.1: This standard gives an introduction to the set ofstandards

802 2: This standard describes the upper part of the DLL


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802.2: This standard describes the upper part of the DLL,

which uses Logical Link Control Protocol. 802.3: Describes the LAN Standard for Ethernet 802.4: Describes the LAN Standard Token Bus 802.5: Describes the LAN Standard Token Ring 802.6: Describes the LAN Standard Distributed Queue Dual Bus

(DQDB) 802.11: Wireless LAN 802.15: Bluetooth 802.16: Wireless MAN

Each standard covers the physical layer and MAC sublayerprotocol


Ethernet refers to cable (the ether)

d i t i d LAN t h l


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dominant wired LAN technology

first widely used LAN technology Simpler, cheap

Kept up with speed race: 10 Mbps 10 Gbps



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LLC

MAC

Network layer

Physical layer

PKT

LLC PKT

MAC LLC PKT MAC



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WIRELESS 802.11


Modes of operation


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Point Coordination Function (PCF) - In the presenceof a base station all communication must go through

the base station, called an access point

Distribution Coordination Function(DCF) - In the

absence of a base station the computers would just

send to one another directly. This mode is now

sometimes called adhoc networking



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Upon correct receipt of the data frame, B responds with anACK frame, terminating the exchange. If A's ACK timerexpires before the ACK gets back to it, the whole protocol isrun again.


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C is within range of A, so it may receive the RTS frame. If itdoes, it realizes that someone is going to send data soon,so for the good of all it desists from transmitting anythinguntil the exchange is completed. From the informationprovided in the RTS request, it can estimate how long thesequence will take, including the final ACK, so it asserts a

kind of virtual channel busy for itself, indicated by NAV(Network Allocation Vector). D does not hear the RTS, butit does hear the CTS, so it also asserts the NAV signal foritself.



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Duration field tells how long the frame and itsacknowledgement will occupy the channel. This field is alsopresent in the control frames and is how other stations managethe NAV mechanism


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the NAV mechanism.

The frame header contains four addresses. The source anddestination are obviously needed. The other two addresses areused for the source and destination base stations for intercelltraffic.

Sequence field allows fragments to be numbered. Of the 16 bitsavailable, 12 identify the frame and 4 identify the fragment.

Data field contains the payload, up to 2312 bytes

Checksum

Management frames have a format similar to that of data

frames, except without one of the base station addressesbecause management frames are restricted to a single cell.

Control frames are shorter still, having only one or twoaddresses, no Data field, and no Sequence field. The keyinformation here is in the Subtype field, usually RTS, CTS, or ACK.



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Link Layer: Introduction

Some terminology: hosts and routers are nodes

link


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5: DataLink Layer 5-226

hosts and routers are nodes

communication channels thatconnect adjacent nodes alongcommunication path are links wired links

wireless links LANs

layer-2 packet is a frame,encapsulates datagram

data-link layer has responsibility oftransferring datagram from one nodeto adjacent node over a link



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Link Layer Services (more)


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Flow Control: pacing between adjacent sending and receiving nodes important if large propagation delays

Error Detection:

errors caused by signal attenuation, noise. receiver detects presence of errors:

signals sender for retransmission or drops frame

Error Correction:

receiver identifies and correctsbit error(s) withoutresorting to retransmission

Half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit,

but not at same time Lt Col Chandan Tiwari

Adaptors Communicating

sending

rcvingdatagram

link layer protocol


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link layer implemented in

adaptor (aka NIC) Ethernet card, PCMCI

card, 802.11 card

sending side:

encapsulates datagram ina frame

adds error checking bits,rdt, flow control, etc.

receiving side looks for errors, rdt, flow

control, etc

extracts datagram, passesto rcving node

adapter is semi-autonomous

link & physical layers

g

nodeframe

nodeframe

adapter adapter



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Error Detection

EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields


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p y g y

Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction


Parity Checking

Single Bit Parity: Two Dimensional Bit Parity:and correct


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Detect single bit errors Detectand correct

single bit errors

0 0


Internet checksum

Goal:detect errors (e.g., flipped bits) in transmitted


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Sender: treat segment contents

as sequence of 16-bitintegers

checksum: addition (1scomplement sum) ofsegment contents

sender puts checksumvalue into UDP checksumfield

Receiver: compute checksum of received

segment

check if computed checksumequals checksum field value:

NO - error detected

YES - no error detected. Butmaybe errors nonetheless?

segment (note: used at transport layer only)


Cyclic Redundancy Check

view data bits, D, as a binary number


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choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that

exactly divisible by G (modulo 2)

receiver knows G, divides by G. If non-zero remainder:error detected!

can detect all burst errors less than r+1 bits widely used in practice (ATM, HDLC)


CRC Example

Want:r


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D.2r

XOR R = nGequivalently:

D.2r = nG XOR Requivalently:

if we divide D.2r byG, want remainder R

R = remainder[ ]D.2rG


Part 3: Link Layer

3.1 Introduction and 3.6 Hubs and switches


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services 3.2 Error detection

and correction

3.3Multiple accessprotocols

3.4 Link layeraddressing

3.5 Ethernet

3.7 PPP


Multiple Access Links and Protocols

Two types of links: point-to-point


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PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) Old-fashioned Ethernet

upstream HFC 802.11 wireless LAN


Multiple Access protocols

single shared broadcast channel


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two or more simultaneous transmissions by nodes:interference collision if node receives two or more signals at the same time

multiple access protocol

distributed algorithm that determines how nodes sharechannel, i.e., determine when node can transmit

communication about channel sharing must use channelitself!

no out-of-band channel for coordination


802.3 MAC Sub Layer Protocol

Preamble Length Data PadCheck-SourceDestination

SO

Bytes 7 1 2/ 6 2/ 6 2 0-1500 0-46 4


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Preamble of 7 bytes each with bit pattern 10101010 Manchester Encoding of this pattern produces 10 MHzsquare wave for 5.6 sec to allow the receiver clock to

synchronise with the senders. 1 byte Start of Frame 10101011 delimiter

6 byte source and destination addresses.

Length indicates number of bytes in data field.

Pad field ensures min frame size is 64 bytes (lessPreamble+SOF)

Checksum field 32 bit CRC

sumAddressAddressF

48 bit address field provide a unique MAC address

Higher order bit(47th bit) 0 for ordinary addresses and


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1 for group/multicast address. 46th bit to distinguishbetween local and global address-2^(48-2).

All bits are 1 for a broadcast address

The MAC ADDRESS is burned into ROM on a network

interface card

24 bits

ROMRAM

24 bits

00-90-F5 - 0C-99-6ASerial NumberVendor Code


Ideal Multiple Access Protocol

Broadcast channel of rate R bps


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1. When one node wants to transmit, it can send atrate R.

2. When M nodes want to transmit, each can send ataverage rate R/M

3. Fully decentralized: No special node to coordinatetransmissions

4. Simple


MAC Protocols: a taxonomy

Three broad classes:


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Channel Partitioning divide channel into smaller pieces (time slots,

frequency, code)

allocate piece to node for exclusive use

Random Access channel not divided, allow collisions

recover from collisions

Taking turns Nodes take turns, but nodes with more to send can take

longer turns


Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access


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access to channel in "rounds" each station gets fixed length slot (length = pkt

trans time) in each round

unused slots go idle

example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6idle


Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple access


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channel spectrum divided into frequency bands each station assigned fixed frequency band

unused transmission time in frequency bands go idle

example: 6-station LAN, 1,3,4 have pkt, frequencybands 2,5,6 idle

frequenc

ybands


Random Access Protocols

When node has packet to send


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transmit at full channel data rate R.

no a prioricoordination among nodes

two or more transmitting nodes collision,

random access MAC protocol specifies: how to detect collisions

how to recover from collisions (e.g., via delayedretransmissions)

Examples of random access MAC protocols: slotted ALOHA ALOHA

CSMA, CSMA/CD, CSMA/CA


Pure (unslotted) ALOHA

unslotted Aloha: simpler, no synchronization, no


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carrier sensing. when frame first arrives

transmit immediately

collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1,t0+1]


Pure Aloha efficiency

P(success by given node) = P(node transmits) .

P(no other node transmits in [p -1,p ] .


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0 0P(no other node transmits in [p0-1,p0]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

choosing optimum p and then letting n -> infty ...

= 1/(2e) = .18


Slotted ALOHA

Assumptions

all frames same sizeOperation


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time is divided intoequal size slots, time totransmit 1 frame

nodes start to transmitframes only atbeginning of slots

nodes are synchronized

if 2 or more nodestransmit in slot, allnodes detect collision

when node obtains freshframe, it transmits in nextslot

no collision, node can sendnew frame in next slot

if collision, noderetransmits frame in eachsubsequent slot with prob.p until success


Slotted ALOHA


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Pros

single active node cancontinuously transmitat full rate of channel

highly decentralized:only slots in nodesneed to be in sync

simple

Cons collisions, wasting slots idle slots nodes may be able to

detect collision in less than

time to transmit packet clock synchronization (used

in a hub-spoke manner)Lt Col Chandan Tiwari

Slotted Aloha efficiency

For max efficiencywith N nodes, find p*

h

Efficiency is the long-runfraction of successful slots

h h d


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Suppose N nodes with

many frames to send,each transmits in slotwith probabilityp

prob that node 1 hassuccess in a slot= p(1-p)N-1

prob that any node hasa success = Np(1-p)N-1

that maximizesNp(1-p)N-1

For many nodes, takelimit of Np*(1-p*)N-1

as N goes to infinity,gives 1/e = .37

when there are many nodes,each with many frames to send

At best:channel

used for usefultransmissions 37%of time!


CSMA (Carrier Sense Multiple Access)

CSMA l b f


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CSMA: listen before transmit:If channel sensed idle: transmit entire frame

If channel sensed busy, defer transmission

Human analogy: dont interrupt others!


All frames must take more than 2t to send so that


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the txn is still taking place when the noise burst gets

back to the sender, otherwise the sender will

incorrectly conclude that the frame was successfully

sent For 10Mbps LAN with max length of 2500m and 04

repeaters 2t=50 micro sec which corresponds to

min frame size of 500 bits. With safety margin it is

chosen to be 512 bits(64 bytes)


Collisions are caused when two adaptors transmit at the

same time (adaptors sense collision based on voltage

diff )


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CS 640 254

A B

A B

differences) Both found line to be idle

Both had been waiting to for a busy line to become idle

A starts attime 0

Message almost

there at time T when

B starts collision!

How can we be sure A knows about the collision?

How can A know that a collision has taken place? There must be a mechanism to insure retransmission on collision


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CS 640 255

As message reaches B at time T

Bs message reaches A at time 2T

So, A must still be transmitting at 2T

IEEE 802.3 specifies max value of 2T to be 51.2us

This relates to maximum distance of2500m between hosts At 10Mbps it takes 0.1us to transmit one bit so 512 bits (64B) take

51.2us to send

So, Ethernet frames must be at least 64B long

14B header, 46B data, 4B CRC

Padding is used if data is less than 46B Send jamming signal after collision is detected to insure all hosts

see collision

48 bit signal

A B

time 0


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CS 640 256

A B

A B

time = 0

time = T

time = 2T

Ethernet uses CSMA/CD protocol


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Nodes are required to wait for random time for

retransmission after collision.

Need to have the randomization interval growexponentially as more and more consecutive collisions

take place.


After collision, time divided into discrete time slots equal

to worst case RTT.


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After first collision, each stn waits 0 or 1 slot times

before trying again.

After second collision, each stn waits for 0,1,2 or 3 slottimes before trying again.

After i collisions, a random number between 0 and 2i1

is chosen.


After 10 collisions, randomization interval frozen at

1023


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1023. Nodes give up after 16 collisions. Left to upper layers

to resolve the issue

The algorithm ensures

low delay when few nodes collide.

collision is resolved in a reasonable interval when

many nodes collide.


CSMA collisions

collisions can still occur:

propagation delay means

spatial layout of nodes


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propagation delay meanstwo nodes may not hear

each others transmission

collision:

entire packet transmissiontime wasted

note:role of distance & propagation delay

in determining collision probability


CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA llisi s detected ithi sh t tim


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collisions detectedwithin short time colliding transmissions aborted, reducing channel

wastage

collision detection: easy in wired LANs: measure signal strengths,

compare transmitted, received signals

difficult in wireless LANs: receiver shut off whiletransmitting

human analogy: the polite conversationalist


CSMA/CD collision detection


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5: DataLink Layer 5-262Lt Col Chandan Tiwari

Taking Turns MAC protocols

channel partitioning MAC protocols:

share channel efficiently and fairly at high load


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share channel efficiently and fairly at high load

inefficient at low load: delay in channel access,1/N bandwidth allocated even if only 1 activenode!

Random access MAC protocols

efficient at low load: single node can fullyutilize channel

high load: collision overhead

taking turns protocols

look for best of both worlds!Lt Col Chandan Tiwari

Taking Turns MAC protocols

Polling:

master node

Token passing:

control token passed from one


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master nodeinvites slave nodesto transmit in turn

concerns:

polling overhead latency

single point offailure (master)

control token passed from onenode to next sequentially.

token message

concerns:

token overhead latency

single point of failure (token)


Summary of MAC protocols

What do you do with a shared media?Ch l P titi i b ti f d


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Channel Partitioning, by time, frequency or code Time Division, Frequency Division

Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD

carrier sensing: easy in some technologies (wire), hardin others (wireless)

CSMA/CD used in Ethernet

CSMA/CA used in 802.11

Taking Turns polling from a central site, token passing


Part 3: Link Layer

3.1 Introduction and

services

3.6 Hubs and switches

3 7 PPP


263/627



and correction

3.3Multiple access

protocols 3.4 Link layer

addressing

3.5 Ethernet

3.7 PPP


MAC Addresses and ARP

32 bit IP dd ss:k l dd


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32-bit IP address: network-layeraddress used to get datagram to destination IP subnet

MAC (or LAN or physical or Ethernet)

address: used to get frame from one interface to another

physically-connected interface (same network)

48 bit MAC address (for most LANs)burned in the adapter ROM


LAN Addresses and ARP

Each adapter on LAN has unique LAN address


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Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)


LAN Address (more)

MAC address allocation administered by IEEE

manufacturer buys portion of MAC address space


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manufacturer buys portion of MAC address space(to assure uniqueness)

Analogy:

(a) MAC address: like Social Security Number

(b) IP address: like postal address

MAC flat address portability can move LAN card from one LAN to another

IP hierarchical address NOT portable depends on IP subnet to which node is attached


ARP: Address Resolution Protocol

Each IP node (Host,

Router) on LAN has

Question: how to determine

MAC address of B


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Router) on LAN hasARP table

ARP Table: IP/MACaddress mappings for

some LAN nodes< IP address; MAC address; TTL> TTL (Time To Live): time

after which addressmapping will be forgotten

(typically 20 min)

MAC address of Bknowing Bs IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137.196.7.23

137.196.7.78

137.196.7.14

137.196.7.88


ARP protocol: Same LAN (network)

A wants to send datagramto B, and Bs MAC address

not in As ARP table

A caches (saves) IP-to-

MAC address pair in itsP bl l f


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not in A s ARP table. A broadcasts ARP query

packet, containing B's IPaddress

Dest MAC address =

FF-FF-FF-FF-FF-FF all machines on LAN

receive ARP query

B receives ARP packet,replies to A with its (B's)

MAC address frame sent to As MAC

address (unicast)

MAC address pair in itsARP table until informationbecomes old (times out)

soft state: informationthat times out (goes

away) unless refreshed ARP is plug-and-play:

nodes create their ARPtables withoutintervention from net

administrator


DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamicallyobtain its IP addressfrom network server when it joins network


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from network server when it joins networkCan renew its lease on address in use

Allows reuse of addresses (only hold address while connected anon

Support for mobile users who want to join network (more shortly)DHCP overview:

host broadcasts DHCP discover msg

DHCP server responds with DHCP offer msg

host requests IP address: DHCP request msgDHCP server sends address: DHCP ack msg


DHCP client-server scenario

223 1 2 1A DHCP


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223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B E

DHCPserver

arriving DHCPclient needs

address in this

network


DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

DHCP discover

src : 0.0.0.0, 68

dest.: 255.255.255.255,67

yiaddr: 0.0.0.0

transaction ID: 654


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time

DHCP offer

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 654

Lifetime: 3600 secs

DHCP requestsrc: 0.0.0.0, 68

dest:: 255.255.255.255, 67

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs


Routing to another LAN

walkthrough: send datagram from A to B via R

assume A knows B IP address


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Two ARP tables in router R, one for each IPnetwork (LAN). R works as a gateway into anothernetwork

A

RB


A creates datagram with source A, destination B A uses ARP to get Rs MAC address for 111.111.111.110

A creates link-layer frame with R's MAC address as dest,frame contains A-to-B IP datagram

As adapter sends frame

Rs adapter receives frame


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R s adapter receives frame

R removes IP datagram from Ethernet frame, sees itsdestined to B

R uses ARP to get Bs MAC address

R creates frame containing A-to-B IP datagram sends to B

A

RB


Part 3: Link Layer

3.1 Introduction and

services

3.6 Hubs and switches

3 7 PPP


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and correction

3.3Multiple access

protocols 3.4 Link layer

addressing

3.5 Ethernet

3.7 PPP


Ethernet

dominant wired LAN technology:

cheap $20 for 100Mbs!

first widely used LAN technology


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first widely used LAN technology

Simpler, cheaper than token LANs and ATM

Kept up with speed race: 10 Mbps 10 Gbps

Metcalfes Ethernetsketch


Star topology

Bus topology popular through mid 90s

Now star topology prevails

Connection choices: hub or switch (more later)


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Connection choices: hub or switch (more later)

hub or

switch


Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or othernetwork layer protocol packet) in Ethernet frame


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Preamble: 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011

used to synchronize receiver, sender clock rates


Ethernet Frame Structure (more)

Addresses: 6 bytes if adapter receives frame with matching destination


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f p f m m gaddress, or with broadcast address (eg ARP packet), itpasses data in frame to net-layer protocol

otherwise, adapter discards frame

Type: indicates the higher layer protocol (mostlyIP but others may be supported such as NovellIPX and AppleTalk)

CRC: checked at receiver, if error is detected, theframe is simply dropped


Unreliable, connectionless service

Connectionless: No handshaking between sending

and receiving adapter


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and receiving adapter. Unreliable:receiving adapter doesnt send acks or

nacks to sending adapter stream of datagrams passed to network layer can have

gaps gaps will be filled if app is using TCP

otherwise, app will see the gaps


Ethernet uses CSMA/CD

No slots

adapter doesnt transmit

Before attempting a

retransmission,


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adapter doesn t transmitif it senses that someother adapter istransmitting, that is,

carrier sense transmitting adapter

aborts when it sensesthat another adapter is

transmitting, that is,collision detection

retransmission,adapter waits arandom time, that is,random access

Minimum Ethernetframe size related tothe physical size ofthe network


Ethernet CSMA/CD algorithm

1. Adaptor receivesdatagram from net layer &

creates frame

4. If adapter detectsanother transmission while

transmitting, aborts and


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creates frame2. If adapter senses channelidle (96 bit times), it startsto transmit frame. If it

senses channel busy, waitsuntil channel idle and thentransmits

3. If adapter transmits

entire frame withoutdetecting anothertransmission, the adapter isdone with frame !

transmitting, aborts andsends 48 bit jam signal

5. After aborting, adapterenters exponential backoff:

after the mth collision,adapter chooses a K atrandom from{0,1,2,,2m-1}. Adapter waits

K512 bit times and returnsto Step 2


Ethernets CSMA/CD (more)

Jam Signal: make sure allother transmitters are aware

of collision; 48 bits

Exponential Backoff:

Goal: adapt retransmissionattempts to estimated


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Bit time: .1 microsec for 10Mbps Ethernet ;for K=1023, wait time is about50 msec

pcurrent load heavy load: random wait

will be longer

first collision: choose K

from {0,1}; delay is K 512bit transmission times

after second collision:choose K from {0,1,2,3}

after ten collisions, chooseK from {0,1,2,3,4,,1023}


CSMA/CD efficiency

Tprop = max prop between 2 nodes in LAN

ttrans = time to transmit max-size frame


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ttrans t me to transm t max s ze frame

Efficiency goes to 1 as tprop goes to 0

Goes to 1 as ttrans goes to infinity

Much better than ALOHA, but still decentralized,simple, and cheap

transpr op tt /51

1efficiency


Manchester encoding


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Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to

synchronize to each other no need for a centralized, global clock among nodes!

Hey, this is physical-layer stuff!Lt Col Chandan Tiwari

Chapter 4

Network Layer


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Part 5: Network Layer

5. 1 Introduction

5.2 Datagram networks

5.5 Routing algorithms

Link state


286/627


5.3 Whats inside a router

5.4 IP: Internet Protocol

Datagram format

IPv4 addressing

ICMP

IPv6

Distance Vector

Hierarchical routing

5.6 Routing in the Internet

RIP OSPF

BGP

5.7 Broadcast and multicast

routing

Network layer transport segment from sending to receiving host

on sending side encapsulates segments intodatagrams

on rcving side, delivers segments to transport layer

network layer protocols in everyhost, router

Router examines header fields in all IP datagramspassing through it networkdata linkph si l

networknetworkd t li k

applicationtransportnetwork

data linkphysical


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networkdata linkphysical


physical



data linkphysical


data linkphysical

applicationtransportnetworkdata linkphysical

forwarding:move packets from routers input

to appropriate router output


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to appropriate router output

routing: determine route taken by packets

from source to dest.

routing algorithms

routing algorithm

local forwarding table

h d l t t li k

Interplay between routing and forwarding


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1

23

0111

value in arriving

packets header

header value output link

0100

0101

0111

1001

3

2

2

1


5. 1 Introduction



Link state


290/627




Datagram format

IPv4 addressing

ICMP

IPv6

Distance Vector



RIP OSPF

BGP


routing

datagram network provides network-layer

connectionless service


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connectionless service

analogous to the transport-layer services,

but:

service: host-to-host

implementation: in network core

Datagram networks

routers: no state about end-to-end connections

no network-level concept of connection

packets forwarded using destination host address packets between same source-dest pair may take different paths


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packets between same source dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetwork

data linkphysical

1. Send data 2. Receive data

Destination Address Range Link Interface

11001000 00010111 00010000 00000000

4 billionpossible entries


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11001000 00010111 00010000 00000000

through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000

through 2

11001000 00010111 00011111 11111111

otherwise 3

Prefix Match Link Interface

11001000 00010111 00010 0

11001000 00010111 00011000 1


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11001000 00010111 00011000 1

11001000 00010111 00011 2

otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?


5. 1 Introduction



Link stateDi t V t


295/627




Datagram format

IPv4 addressing

ICMP

IPv6

Distance Vector



RIP OSPF

BGP


routing

The Internet Network layer

Host, router network layer functions:

Transport layer: TCP, UDP


296/627


forwardingtable

Routing protocolspath selectionRIP, OSPF, BGP

IP protocoladdressing conventionsdatagram formatpacket handling conventions

ICMP protocolerror reportingrouter signaling

Link layer

physical layer

Network

layer


5. 1 Introduction



Link stateDi t V t


297/627




Datagram format

IPv4 addressing

ICMP

IPv6

Distance Vector



RIP OSPF

BGP


routing

ver length

32 bits

16-bit identifierheadertime to

IP protocol versionnumber

header length(bytes)

max number

for

fragmentation/reassembly

total datagramlength (bytes)

head.len

type ofservice

type of data

flgsfragment

offsetupper


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data(variable length,

typically a TCPor UDP segment)

headerchecksum

time tolive

32 bit source IP address

max numberremaining hops

(decremented ateach router)

upper layer protocolto deliver payload to

upperlayer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers

to visit.

how much overhead

with TCP?

20 bytes of TCP

20 bytes of IP

= 40 bytes + app

layer overhead

IP Fragmentation & Reassembly

network links have MTU

(max.transfer size) - largest

possible link-level frame.

different link types, different

MTUfragmentation:

l


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MTUs

large IP datagram divided

(fragmented) within net

one datagram becomes

several datagrams

reassembled only at final

destination

IP header bits used to identify,

order related fragments

in: one large datagramout: 3 smaller datagrams

reassembly

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

One large datagram becomes

Example

4000 byte datagram


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ID

=x

offset

=0

fragflag

=1

length

=1500ID=x

offset=185

fragflag=1

length=1500

ID

=x

offset

=370

fragflag

=0

length

=1040

One large datagram becomesseveral smaller datagrams MTU = 1500 bytes

1480 bytes indata field

offset =1480/8


5. 1 Introduction



Link stateDistance Vector


301/627




Datagram format

IPv4 addressing

ICMP

IPv6

Distance Vector



RIP OSPF

BGP


routing

IP Addressing: introduction

IP address: 32-bit

identifier for host, router

interface

223.1.1.1

223.1.1.2223.1.1.4 223.1.2.9

223.1.2.1


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interface: connection

between host/router and

physical link

routers typically havemultiple interfaces

host typically has one

interface

IP addresses associated

with each interface

223.1.1.3223.1.2.2

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Subnets

IP address:

subnet part (high order

bits)

host part (low order bits)

223.1.1.1

223.1.1.2223.1.1.4 223.1.2.9

223.1.2.1


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host part (low order bits)

Whats a subnet ?

device interfaces with

same subnet part of IPaddress

can physically reach each

other without intervening

router

223.1.1.3223.1.2.2

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

Subnets 223.1.1.0/24 223.1.2.0/24

Recipe

To determine the subnets,detach each interface


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223.1.3.0/24

detach each interface

from its host or router,

creating islands of isolated

networks. Each isolated

network is called a

subnet.

Subnet mask: /24

Subnets

How many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.1.2


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223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

CIDR:Classless InterDomain Routing

subnet portion of address of arbitrary lengthaddress format a b c d/x where x is # bits in


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address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Q: How does networkget subnet part of IP addr?

A:gets allocated portion of its provider ISPsaddress space


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address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23

... .. . .

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Q: How does hostget IP address?


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hard-coded by system admin in a file

/etc/network/interfaces

DHCP:Dynamic Host Configuration Protocol: dynamically getaddress from as server

plug-and-play

Organization 0

Hierarchical addressing allows efficient advertisement of routinginformation:


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Send me anythingwith addresses

beginning200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-UsSend me anythingwith addressesbeginning199.31.0.0/16

200.23.20.0/23Organization 2

...

...

ISPs-R-Us has a more specific route to Organization 1

200 23 16 0/23

Organization 0


310/627


Send me anythingwith addressesbeginning

200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organiza

basic nw complete_chandan

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